Vaporization of polychlorocyclohexanes



y 3 1956 A. P. GIRAlTls ETAL VAPORIZATION OF POLYCHLOROCYCLOHEXANESFiled Feb. 29, 1952 \9 09 n V R R m mm 3w mm m w vm vm mm g kw a n an Av w mm 9w nu R mm 8 o 3 V S Y \QJ R s mz mmm y mP E w M RHA m 0 8 T momA 40m Y B VAPORIZATION F POLYCHLOROCYCLO- HEXANES Albert P. Giraitis,David D. Humphreys and Thomas A. Leeper, Baton Rouge, La., assignors toEthyl Corpora tion, New York, N. Y., a corporation of DelawareApplication February 29, 1952, Serial No. 274,111

4 Claims. (Cl. 260-648) This invention relates to thedehydrochlorination of oragnic chlorine compounds and more particularlyto a new and improved method for the vapor phase dehydrochlorination ofpolychlorocyclohexanes, such as benzene hexachloride and similarcompounds.

The dehydrochlorination of benzene hexachloride to producetrichlorobenzene has previously been carried out at elevatedtemperatures, usually in the presence of iron or ferric chloride. Otherimproved methods have recently been developed using novel catalystswhich provide more isomer selectivity, for example, for selectiveformation of the valuable 1,2,4-trich1orobenzene isomer. The latterisomer is useful as an intermediate in the production of valuable higherchlorinated derivatives. It may be chlorinated, for example, to producel,2,4,5-tetrachlorobenzene, the latter hydrolyzed to provide 2,4,5-trichlorophenol, and this product further treated with acetic acid toobtain 2,4,5-trichlorophenoxyacetic acid, commonly known as 2,4,5-T.

The preferred methods for dehydrochlorinating benzene hexachloride arecarried out in the vapor state. However, vaporization of the benzenehexachloride results in undesired coking (carbon deposition) whichcauses fouling of process equipment and the buildup thereon of a heatinsulating surface layer. Thus, frequent interruptions in operation arenecessitated to assure a satisfactory flow of reactants through theprocess equipment and to maintain the desired heat transfer through thevaporizer walls to effect vaporization of the benzene hexachloride. Inconsequence, operational costs of the vapor phase processes have beenrelatively high, and continuous operation of the process has beenimpractical.

In general, it has been found desirable to melt the benzenehexachloride, vaporize the same, and dehydro chlorinate the vapors inseparate steps. While all three operations, and particularly the lattertwo steps, may be carried out simultaneously in the same vessel, the useof separate operations facilitates control of residence times andtemperature conditions to minimize precrack ing and undesired coking ofthe benzene hexachloride. Normally, the residence time in the melt potshould be as short as possible, consistent with providing an amplesupply of molten benzene hexachloride for vaporization so as to minimizeprecracking, and the residence time and vaporization rate should becontrolled within relatively close limits to prevent both coking andprecraclc ing of the benzene hexachloride to trichlorobenzene in thevaporizer.

It is therefore an object of the present invention to provide a methodfor dehydrochlorinating polychlorocyclohexane compounds in which thelatter compound may be vaporized and subsequently dehydrochlorinatedwhile eliminating or at least greatly minimizing the deposition of cokeon the surfaces of the process equipment.

Another object of the present invention is to provide a method of theabove type characterized by providing nited States Patent 0 2,757,2liPatented July 31, 1956 'ice a high degree vaporization per unit volumeof vaporizer space.

Another object is to provide a process which will allow the maintenanceof the polychlorocyclohexane compound in a molten state for relativelylong periods of time without appreciable precracking thereof.

Still another object of this invention is to provide a method of theabove type for benzene hexachloride which will permit occasionalstarting or stopping of the process without solidification of normallysolid polychlorocyclohexane compound within the reaction equipment.

Other objects and advantages of the present invention will becomeapparent as the description proceeds especially when considered inconnection with the accompanying drawing wherein:

The figure is a schematic flow diagram of one mode ofdehydrochlorinating benzene hexachloride to produce trichlorobenzeneembodying the features of the present invention.

Before explaining the present invention in detail, it is to beunderstood that the invention is not limited in its application to theprocess specifically described herein, since the invention is capable ofother embodiments and of being practiced or carried out in various ways.Also it is to be understood that the phraseology or terminology employedherein is for the purpose of description and not of limitation. a

The process of the present invention comprises dehydrochlorinating in avapor state a polychlorocyclohexane, such as benzene hexachloride, toproduce trichlorobenzene. An essential feature of this invention bywhich the above objects are accomplished is the addition of a quantityof trichlorobenzene to the polychlorocyclohexane to be vaporized so asto facilitate melting and vaporization thereof, and, in consequence, toprevent precracking and coking of the polychlorocyclohexane.

As noted above, it is preferred to melt and vaporize thepolychlorocyclohexane in separate vessels, prior to effecting catalyticcracking in a reaction vessel. The trichlorobenzene is preferably addeddirectly to the polychlorocyclohexane in the melt pot, thus permittinglower temperatures therein. Since in actual operation it is normallynecessary to retain a relatively large volume of the moltenpolychlorocyclohexane for feeding to the vaporizer, resulting in longresidence periods, it is highly desirable to maintain as low atemperature in the melt pot as reasonably possible. At normal meltingtemperatures of benzene hexachloride, for example, with the residencetimes generally encountered, considerable precracking thereof isfrequently experienced. At the lower temperatures permitted by theaddition of trichlorobenzene, such precracking is eliminated or materialreduced.

Since the 1,2,4-trichlorobenzene isomer is most valuable at this timefor upgrading purposes, it is generally desired to utilize processconditions, including temperatures and catalysts, which favor1,2,4-trichlorobenzene formation. In such cases, it is also desired toadd to the polychlorocyclohexane to be dehydrohalogenated the sametrichlorobenzene isomer. In actual operation, this is accomplished bymerely recycling a portion of the trichlorobenzene product of thecracking operation to the melt pot for blending with thepolychlorocyclohexane feed. 1

The quantity of trichlorobenzene which should be added to thepolychlorocyclohexane is largely dependent upon the temperature desiredin the melt pot, and the temperature and degree of vaporization of thepolychlorocyclohexane desired in the vaporizer. The trichlorobenzeneacts as a vapor carrier for the polychlorocyclohexane in the vaporizerand, at similar temperature conditions, materially increases the degreeof vaporization of the polychlorocyclohexane. In normal operation, thetemperature of the vaporizer is generally lowered, thus eliminating ormaterially reducing both precracking and coking therein, and theresidence time of the material in the vaporizer. The latter is one ofthe maior factors controlling coke formation.

In general, the concentration of trichlorobenzene in the feed solutionmay vary between about and 70 percent by weight. Normally, it ispreferred to maintain a concentration of trichlorobenzene between about30 and 40 percent by weight of the 'total feed stream. The feedcomposition obviously controls to some extent both the temperature inthe melt pot and in the vaporizer. When employing a feed consisting of/3 trichlorobenzene and benzene hexachloride, for example, it isfrequently desired to maintain a temperature of about 150 C. in the meltpot and about 300 C. in the vaporizer.

Catalysts suitable for the present process are well known and includemetallic iron, ferric chloride, aluminum chloride, and similarcompounds. Activated carbon has recently been found to be highlyeffective and preferred in employing the present process.

The addition of trichlorobenzene to the benzene hexachloride, to bedehydrochlorinated, also permits easy and eflicient starting andstopping of a continuous process without danger of solidification of thenormally solid benzene hexachloride in the process equipment. Apreferred method of operation comprises initially filling the entireprocess equipment, including the melt pot, vaporizer and reactor withthe normally liquid trichlorobenzene. The benzene hexachloride or otherpolychlorocyclohexane is then gradually added to the melt pot astemperature conditions within the process equipment approach normaloperating conditions. Conversely, when it becomes necessary to shut downthe process for any reason, it is preferred to gradually replace thepolychlorocyclohexane in the melt pot with trichlorobenzene so as toeliminate the possibility of solidification of the relatively highmelting polychlorocyclohexanes within the process equipment.

With reference to the drawing and considering steady state conditions,benzene hexachloride is melted with trichlorobenzene in a. melt pot 20,the trichlorobenzene being preferably obtained as a product recycle, andthe liquid mixture is continuously fed into a knockout drum 3%). Thismolten mixture is immediately heated and is continuously circulated fromthe knockout drum through the vaporizer 40 and back to the knockout drumWhere a controlled quantity of the benzene hexachloride-trichlorobenzenemixture vapor is flashed. The latter vapor fraction is then furtherheated in a superheater 50 and passed into a reactor 60, wherein thebenzene hexachloride vapors are cracked to trichlorobenzene. The producttherefrom, essentially pure trichlorobenzene, is then liquefied incondenser 70 and collected in a trichlorobenzene holdup drum 80. Theliquid product may then be withdrawn therefrom as desired. A portion ofthe molten mixture in the knockout drum 30 is continuously circulatedthrough a filter feed tank 90 and a coke filter 100 to remove traces ofsolid carbon particles formed during the heating operations.

More specifically the melt pot 2.0 is connected at its top with thehopper i0 and with the trichlorobenzene holdup drum 80 through lines 11and 81 respectively. The melt pct 20 is also vented to atmospherethrough line 23. The molted mixture is circulated from the bottom of themelt pot 20 to the top of the knockout drum 30 through lines 24 and 26by means of pump 28. Valves 25 and 27 are provided in the lines 24 and26, respectively.

The knockout drum 30 is connected at its bottom with the bottom of thevaporizer 40 through lines 31 and 33 and pump 32, and also with thefilter feed tank 90 through line 34. The outlet or upper end of thevaporizer 40 is connected with the upper end of the knockout drum 36through line 41.

The flashed vapor fraction containing benzene hexa chloride andtrichlorobenzene, in feed ratio, is then passed through line 35 to thesuperheater 50. The latter is connected through line 51 and valve 52 tothe bottom of the reactor 60. Lines 61 and 71 connect the reactor 60 tothe condenser 70 and the condenser to the trichlorobenzene holdup drum80, respectively. A valve 62 is also provided in the line 61.\Vithdrawal of product is accomplished through line 31 and valve 82.

The filter feed tank is connected through the coke filter to the meltpct 20, by means of lines 91, 93 and 101 and pump 92. Valves 94 and 95are provided in the lines 91 and 93, respectively.

The melt pot 20, knockout drum 30, trichlorobenzene holdup drum 80, andfilter feed tank 90 are preferably simple cylindrical drums. If desired,the melt pot may be provided with internal agitation, and the knockoutdrum may be provided with insulation and/or jacketed.

The vaporizer 40, superheater 50, reactor 60, and condenser 70 are allpreferably tube bundles, i. e., series of parallel tubes. The reactortubes generally are packed with a suitable catalyst, such as activatedcarbon, and a screen is normally provided at the lower inlet ends of thecatalyst-filled tubes.

The following is an example of one preferred mode of carrying out theprocess of the present invention.

The apparatus illustrated schematically in the drawing was first filledwith a solution comprising essentially trichlorobenzene. This solutionwas continuously circulated through the system. The melt pot andvaporizer were heated externally and benzene hexachloride was graduallyadded to the melt pot 20 from the hopper 10 as the system approachedoperating temperatures. The benzene hexachloride had the followingcomposition in percent by weight:

Alpha isomer 77.5 Beta isomer 9.9 Gamma isomer 7 .0 Epsilon isomer 1.0Heptachlorocyclohexane 2.6 Tetrachlorocyclohexane 0.3

After the system attained a steady state continuous operation, thebenzene hexachloride-trichlorobenzene weight feed ratio was 2: 1. Thebenzene hexachloride was fed to the melt pot at a rate of about 10 gramsper minute and the melt pot was maintained at a temperature of about C.This mixture was continuously fed to the top of the knockout drum fromwhence the same was circulated through the vaporizer 40. The temperaturein the knockout drum and vaporizer were maintained at about 285 C.Turbulent flow is maintained through the vaporizer and approximately onepercent of the benzene hexachloride was flashed per pass. Thecomposition of the liquid circulated into the vaporizer was about 90percent benzene hexachloride and 10 per cent trichlorobenzene whereasthe flashed vapor had a composition essentially the same as the feedcomposition66.7 percent benzene hexachloride and 33.3 percenttrichlorobenzene. The flashed vapor was passed through the superheater50 wherein the temperature was raised to about 350 C. These superheatedvapors were then passed through the reactor 60 wherein the benzenehexachloride vapors were dehydrohalogenated to form trichlorobenzene,using an activated charcoal catalyst. The trichlorobenzene product wascondensed at approximately 90 C. and recovered in the trichlorobenzeneholdup drum 80.

A portion of the liquid phase in the knockout drum was circulatedthrough a coke filter to remove quantities of coke which formed in thevaporization and the filtrate was returned to the melt pot. A sufficientquantity was circu- Percent 1,2,4-trichlorobenzene 791,2,3-trich1orobenzene 9 Pentachlorocyclohexene 3 Dichlorobenzenes andtetrachlorobenzenes 9 As is believed apparent from the foregoing, thepresent invention eliminates or materially reduces the problem of cokeformation and accumulation of coke in the melting and vaporization ofpolychlorocyclohexanes, such as benzene hexachloride prior todehydrochlorination thereof by the addition of controlled quantities oftrichlorobenzene to the polychlorocyclohexane compound. Thetrichlorobenzene additive lowers both the melt temperature andvaporization temperature and, in addition, materially aids thevaporization rate of the polychlorocyclohexane compound. Thus, theresidence time in the melt pot and particularly in the vaporizer ismaterially reduced, in consequence substantially eliminating or greatlyminimizing coke formation and precracking of the feed material.

We claim:

1. In the process of melting and vaporizing a polychlorocyclohexanecompound prior to dehydrochlorination thereof under vapor phase reactionconditions, the novel steps of liquefying said polychlorocyclohexanecompound in the initial presence of above about 5 percent by weight oftrichlorobenzene based on the weight of said polychlorocyclohexane, andthereafter heating said liquid mixture to a temperature of at least theboiling point of the mixture to efiect vaporization of thepolychlorocyclohexane and trichlorobenzene.

2. The process of claim 1 wherein said polychlorocyclohexane compound isbenzene hexachloride.

3. The process of claim 1 wherein said trichlorobenzene is present inthe mixture in an amount between about and percent by weight of thepolychlorocyclohexane compound.

4. In a process for melting and vaporizing benzene hexachloride prior tothe vapor phase dehydrochlorination of said benzene hexachloride toproduce a preponderant amount of 1,2,4-trichlorobenzene, the novel stepsof heating benzene hexachloride in the initial presence of1,2,4-trichlorobenzene to a temperature of at least the melting point ofthe mixture, said trichlorobenzene being present in an amount betweenabout 5- by Weight of the mixture, and thereafter heating said meltedmixture to at least the boiling point thereof to efiect the vaporizationof benzene hexachloride and trichlorobenzene.

References Cited in the file of this patent FOREIGN PATENTS 955,816France July 4, 1949

1. IN THE PROCESS OF MELTING AND VAPORIZING A POLYCHLOROCYCLOHEXANECOMPOUND PRIOR TO DEHYDROCHLORINATION THEREOF UNDER VAPOR PHASE REACTIONCONDITIONS, THE NOVEL STEPS OF LIQUEFYING SAID POLYCHLOROCYCLOHEXANECOMPOUND IN THE INITIAL PRESENCE OF ABOVE ABOUT 5 PERCENT BY WEIGHT OFTRICHLOROBENZENE BASED ON THE WEIGHT OF SAID POLYCHLOROCYCLOHEXANE, ANDTHEREAFTER HEATING SAID LIQUID MIXTURE TO A TEMPERATURE OF AT LEAST THEBOILING POINT OF THE MIXTURE TO EFFECT VAPORIZATION OF THEPOLYCHLOROCYCLOHEXANE AND TRICHLOROBENZENE.